Caption: Viagra impotence pills. Boxes and blister packs containing Viagra pills for the treatment of impotence. The pills contain the drug sildenafil citrate. The pack on the left contains 50 milli- gram (mg) pills, the one on the right 100mg pills. Viagra does not cause penile erections directly, but instead affects the response to sexual stimulation. It enhances the smooth muscle relaxant effects of nitric oxide, a substance normally released in response to sexual stimulation. Smooth muscle relaxation allows blood to enter the penis, leading to an erection.